Decanter centrifuge

ABSTRACT

A decanter centrifuge (1) has a rotor comprising a helical conveyor (2) and a drum (20). The conveyor (2) comprises a conveyor hub (3) and helical flights (4), and the drum comprises an inner shell (21) of steel and an outer shell (22) of fiber reinforced plastic. The conveyor (2) consists of a shaft of tube form (11), primarily made from carbon fiber reinforced resin, and helical flights (4), primarily made from polyurethane. The flights (4) of the helical conveyor are formed in such a way that they all the time is in contact with the inner periphery of the drum (31). The division of the drum in an inner and an outer shell causes the mass of the drum to be reduced considerably.

This application is the national phase of international applicationPCT/DK95/00440 filed Nov. 6, 1995 which designated the United State.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This is invention relates to a decanter-type centrifuge for theseparation of suspended solids from a liquid medium, the centrifugecomprising a drum and a helical conveyor rotatable mounted therein, saidhelical conveyor having a conveyor hub and at least one helical flight.

2. Prior Art

Conventionally, a decanter-type centrifuge (hereinafter referred to as adecanter centrifuge) comprises a hollow drum of cylindrical/conicalcross-section rotatably supported by bearings and having a helicalconveyor therein, rotatably supported by bearings relative to the drum.Such a centrifuge is primarily used for the separation of solidparticles from sludge, i.e. sludge from sewage treatment plants.

The centrifuge works by having the materials to be separated introducedwithin the drum through a pipe along the conveyor's axis of rotation viaan inlet arrangement. As the centrifuge rotates, the introduced sludgeforms a toroidal shaped volume along the inner wall of the drum. Byaction of the centrifugal forces, the solid particles are concentratedas a layer along the inner wall of the drum, and from there they aretransported by the helical conveyor towards one end of the centrifuge.Typically, the end of the conveyor is formed as a frustro-cone with anarrow end having approximately the same diameter as the inner diameterof the toroidal-formed sludge volume, whereby the solids leave thecentrifuge having a comparatively higher concentration of solids thanthe incoming sludge. At the end of the centrifuge, the cleaned liquidphase leaves the centrifuge through holes or special extraction means,such as a paring device.

A centrifuge has a limited peripheral speed, fixed by materialproperties and stresses created by the rotation, and the internaltoroidal volume is limited by the maximum length of the drum, whichlimit is primarily governed by the tendency of increased vibrations asthe operational speed gets close to a critical frequency of vibration.Critical vibration frequencies are a property, mainly fixed by thestiffness-to-weight ratio of a body. The lowest ratio for the parts of adecanter centrifuge is found at the helical conveyor.

Known decanter centrifuges often have longitudinally mounted stripsalong the inner wall of the drum, intended to protect the inner wallfrom wear by the solids in the following way. By the action of thecentrifugal forces, a layer of solid particles is deposited on the wall,which layer will be out of reach of the helical conveyor and held inposition against rotation relative to the drum wall by the strips. Bythis method, some degree of self-sealing between the helical edge andthe fixed layer of solid particles will be created.

The capacity of a decanter centrifuge is mainly dependent on twoproperties: the maximum safe operational rotational speed, and the sizeof the toroidal volume of liquid and solids contained in the drum.

The functional lifetime of a decanter centrifuge is limited by wear fromthe solids being conveyed, partly caused by the friction created by thetransport action itself, and partly caused by friction between theperipheral edge of the conveyor against the hard and often sharpparticles concentrated at high density between the strips along the drumwall during the operation of the centrifuge.

As the flights of the helical conveyor are worn along their edges, theeffective volume of separation is reduced accordingly, thus reducing theseparation capacity of the centrifuge.

Decanter centrifuges of the foregoing type are known to have severaldifferent design features and variations.

The limitations originating from critical frequencies and vibrationshave given rise to several complicated designs, e.g., letting thehelical conveyor be supported by the medium to be treated instead ofbeing supported in rigid bearings.

Danish Patent No. 15450 shows a decanter centrifuge with a helicalconveyor comprising a hollow hub with flights having an overall densityless than the density of the lighter phase of the medium to be treated.In this way, the influence of the stiffness/weight ratio of the conveyoron the tendency to create vibrations is eliminated, thus making itpossible to increase the safe operational speed of the centrifuge.

The disadvantages of this arrangement in a centrifuge are that thebearings supporting the conveyor are flexible, thereby making itdifficult to transfer the necessary torque and forces to the conveyorfrom the drive system, thus limiting the conveying capacity.Furthermore, the risk of having deposition of the separated materialalong the inner wall of the drum in a non-coaxial manner is increased,thus causing the centrifuge to be prone to vibrations.

A large number of inventions have been made to deal with wear problems,and most of these have attempted to improve the wear resistance inhighly loaded wear zones.

The latest approaches have been oriented towards the flights of theconveyor. WO 93/22062 describes a decanter centrifuge with helicalflights that have wear resistant rubber protection mounted at theirperipheral edges in such a manner that the rubber profile seen in axialcross-section has a different angle to the axis than the flightsthemselves.

SUMMARY OF THE INVENTION

The aim of the present invention is therefore to provide adecanter-centrifuge of the type having a drum and a rotatable helicalconveyor mounted therein which is safe in operation, which, with mainlythe same dimensions, has a larger separation capacity than known before,and which moreover is simple and inexpensive to manufacture.

These advantages are obtained through characteristic properties of theinvention, in which at least the helical flight of the conveyor ismanufactured from an elastomeric material, e.g., polyurethane.

As described above, in order to increase the separation capacity of acentrifuge, it is necessary to increase the length and/or the rotationalspeed of the rotor. A decanter centrifuge according to the invention canincrease both the length and the rotational speed without sacrificingthe technical safety of operation.

The length increase is possible because both the drum and the conveyorare manufactured from materials that are relatively light and stiffcompared to conventional materials, thus improving the ratio ofstiffness to weight.

The hub of the conveyor, including the feed inlet for the sludge, is ina preferred embodiment made of the same material as the helical flight,and the stiffness of the conveyor is increased by a cast-in pipe,reaching from one end of the conveyor to the other between the bearingswhich support the conveyor. The combination of the light materials usedfor the hub and flight, and the light and stiff material of the pipe,produces a conveyor of a previously unknown stiffness-to-weight ratio,by which a considerable increase of the conveyor's length and/or aconsiderable increase of the 1st critical frequency of the conveyor ispossible.

As a consequence of an increased rotational speed, conventional rotorsare prone to increased friction and wear of the helical flights againstthe inside wall of the drum, resulting in a decreasing lifetime of thecentrifuge. This is primarily caused by the deposited layer of heavy andhard particles between the strips. Further, the pressure between theperipheral edge of the helical flights and the deposited particlesbecomes very large, further increasing the resistance and wear.

The present invention is distinguished, however, by the fact that thisproblem does not arrive, even at very high rotational speeds. The verycharacteristic that the helical flights of the conveyor are made from aflexible material and at the same time are in contact with the innerwall of the drum prevents the formation of a "protection" layer betweenthe peripheral parts of the flights and the inside of the drum. Thus, noheavy, hard particles can be deposited and retained between the flightsand the inside wall of the drum, and the high pressures creating wearare not present. Further, the flexible material of the flights yields toparticles that may be trapped, thus preventing excessive wear.

By letting the helical flights be in contact with the inner wall of thedrum, an increase in operational safety is obtained, as solid matter isno longer permitted to be deposited non-coaxially so as to causevibrations. By letting the helical flights be composed of an elastomer,and by making them to have an angle relative to the inner wall of thedrum different from 90 degrees, it is a result that the flights, evenafter some wear at the edge, will be in contact with the inside wall ofthe drum, and at the same time, the wear of the flights will bedecreased considerably, because they are in contact with a smooth wallinstead of a layer of deposited, hard particles.

Furthermore, the separation volume of the centrifuge will be unchangedthroughout its lifetime, and the same is true for its separationcapacity.

To further decrease the wear of the flights and increase the lifetime ofthe centrifuge, the profile of the inner wall in cross-sectional view isformed as a gradually converging transition from a cylindrical outlineat the liquid inlet end to a conical outline at the solids' outlet fromthe drum whereby at every point along the profile, the wear inducingforces are minimized, in particular in the most critical places, i.e. atthe feed introduction point.

Furthermore, considerable savings are obtained regarding manufacturing,as the conveyor is castable in a simple mold, and intensive machiningprocesses have been eliminated or replaced by modern fiber technology.

In a preferred embodiment, the conveyor hub and the helical flights aremade of only one material. This requires a very large-diameter hub, andthus results in a design with little stiffness, but one whichincorporates all the manufacturing advantages of the invention.

Therefore, the preference will normally be to add stiffness to thedesign by incorporating a stiffener in the form of a pipe connecting thebearings and made of a material having great stiffness in relation toits weight. The helical conveyor flights are added as a cast-on featurecomprising a material having a density close to the density of theliquid phase of the material to be treated, causing a buoyancy force onthe submerged flights of the same magnitude as the mass forces from theflights' material, when the centrifuge operates.

The combined effect of this is that the ratio of mass of the conveyor ina submerged condition to the bending stiffness of the conveyor supportedat the bearings is decreased, whereby the first critical vibrationfrequency of the conveyor is increased.

The flights of the conveyor are, for this and other reasons, primarilymade from polyurethane.

The helical flights of the conveyor may, in cases where large loads areoccurring in the transport of deposited material along the inner wall ofthe drum, be reinforced by cast-in plates or lamellas.

Such reinforcing members are preferably made of fiber-reinforced resinsin order to reduce the mass/stiffness ratio of the reinforcement.

Friction and wear are not totally eliminated, and over time the wear ofthe peripheral edge of the helical flights progresses. In order not tolose the advantages associated with the fact that the flights are incontact with the inner wall of the drum, the flights of one preferredembodiment are formed in such a way that the transporting face of theflights is at an angle to the profile of the inner wall of the drum, asseen in axial cross-section, of more than 90 degrees.

Accordingly, the helical flights, at their innermost position closest tothe axis, are formed in such a way that they can pivot around the pointof attachment to the hub.

By having this feature, wear on the peripheral edge of the flights willcause the flights to pivot outward by action of the centrifugal force,until they reach contact with the inner wall. The angle between theflights and the profile of the inner wall will diminish, but this willnot change the separation capability of the centrifuge significantly.

The pivoting action will be assisted by the pressure created by thesolid material on the transport side of the flights, increasing thesealing action between the flights and the inner wall of the drum.

The above characteristics of the present invention are particularlyadvantageous in the converging part of the decanter centrifuge, wherethe deposited solids are transported out of the drum. In this part ofthe centrifuge, leakage between the peripheral edge of the flights andthe inner wall will cause the solids to slide backwards towards thecylindrical part of the drum and consequently not be conveyed out.

This disadvantage is further diminished when the "hill", by which thesolids are carried upwards towards the solids' outlet, is formed with asmaller angle of "elevation" in the areas where the centrifugal forcesare at maximum, that is at the "foot of the hill" between thecylindrical and converging parts of the drum.

Therefore, in a preferred embodiment, the profile of the inner wall ofthe drum is made in three sections along the axis, comprising a firstcylindrical part in the liquid outlet end, a second conical part with asurface angle a, and a third conical part with a surface angle p, whichis greater than a.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in detail referring tothe attached drawings, where

FIG. 1A shows an axial cross-section through a helical conveyor of adecanter centrifuge according to the invention,

FIG. 1B is an enlarged detailed view of a portion of a helical flight ofthe conveyor shown in FIG. 1A;

FIG. 2 is an axial cross-section through a drum of a decanter centrifugeaccording to the invention, and

FIG. 3 is an axial cross-section through an alternative embodiment of adecanter centrifuge according co the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1A illustrates a conveyor 2 for a decanter centrifuge (shown inFIG. 3). The conveyor 2 comprises a conveyor hub 3 and helical flights4. The conveyor hub 3 and the helical flights 4 are all made of the sameflexible material and are cast in one piece. The flexible material ispolyurethane. Other materials having similar properties, i.e. densityand wear resistance, with the ability to function at a satisfactorylevel in a decanter centrifuge according to the invention, may also beemployed.

The conveyor hub 3 extends from a foremost end 5 to an rear end 6, andis connected to shafts 7, 8 by respective bearings 9, 10. The shafts 7and 8 are made of steel, and an intermediate stiffener 11, made from afiber reinforced resin material, extends the length of the conveyor hub3. An opening 12 protrudes through the conveyor hub 3. The rear shaft 8and its extension 13 are hollow and are fastened to the hollowintermediate stiffener 11. The hub 3 is also hollow, so that the mediumto be treated can be introduced from hub 3 into the interior of thecentrifuge through the opening 12.

As shown in FIG. 3, the rear, hollow shaft 8 is connected at its freeend via a rotating seal 35 to a piping system (not shown) for supplyingmedium to be treated to hub 3.

Upstream of the opening 12, the flexible material, of which the conveyorhub is manufactured, is dimensioned to the full diameter d of the hub,along a distance a1. Apart from the distance a1, the hub 3 is hollowthroughout.

The helical flights 4 extend from the outer periphery 14 of the conveyorhub 3 to the flight's outer peripheral edge 15. The helical flights 4form two continuous helixes, exceeding from the rear end 6 of theconveyor hub to its foremost end 5. The helical flights 4 form an angleΥ relative to the outer periphery 14 of the conveyor hub 3, which angledecreases gradually from approximately 90 degrees from the rear end 6 ofthe conveyor hub to the foremost end 5 of the hub 3.

The helical flights are in the embodiment shown able to pivot through atransition point P between the outer periphery 14 of the hub 3 and aninner edge area 16 of the flights 4. An enlarged view (FIG. 1B) showshow the helical flights may be reinforced by the introduction of cast-instiffeners in the form of lamellas 17.

The helical flights 4 are made as two continuous helixes in order tocreate ideal dynamical balance. A number other than two may be chosen,provided that proper balancing devices are employed.

The outer diameter D of the helical flights 4 is constant along a firstaxial distance a3 from the rear end of the conveyor hub, but thendecreases linearly along a second distance a4 towards the foremost end 5of the conveyor hub 3, and then further decreases along a third distancea5. The conveyor hub likewise decreases from a diameter d and itsrearmost end towards the foremost end of the hub 3.

FIG. 2 illustrates a drum 20 for a decanter centrifuge according to theinvention. The drum 20 comprises an inner shell 21 made from steel andan outer shell 22 made from fiber reinforced resin.

A rear end 23 of the inner shell 21 ends in a flange 24 with means 25for fastening this flange 24 to another flange (not shown) that providesthe bearing support for rotation at this end. The opposite end 26 theinner shell 21 is provided with openings 27, 28 for the outlet of thesolid phase of the medium to be treated in the centrifuge. The innershell 21 ends up at end 26 in a flange 29 with means 30 for fasteningthis flange to another flange (not shown) which provides the bearingsupport for rotation at this end (FIG. 1A). The inner shell 21 is hollowall through, so that the conveyor 2 can be accommodated into the drum20.

As mentioned above, the outer shell 22 is made from a fiber-reinforcedresin and is intended to provide stiffness and strength to the innershell 21. On its inside 31, the inner shell is provided with awear-resistant surface coating. The inner shell 21 has an insidediameter D equal to the outside diameter D of the helical flights 4(FIG. 1A), and D is constant along a first axial length a6. Along asecond axial length a7 following a6, the inner shell 21 has a conicalsection with a cone angle a of 4 degrees, and along a third axial lengtha8 the inner shell is likewise conical, but with a cone angle β of 8degrees.

FIG. 3 illustrates the assembled decanter centrifuge 1 according to theinvention comprising the conveyor 2 and drum 20 as illustrated in FIGS.1A and 2, respectively. Further to this, the decanter centrifugecomprises a supporting structure of known type and driving means (notshown). The density of the material forming the helical flights 4 isapproximately equal to, but slightly larger than the density of theliquid phase of the medium to be treated in the centrifuge, assuringthat the outer edge of the helical flights 4 is always in contact withthe inside surface 31 of the drum 20.

The front side 32 of the helical flights 4 is angled by an anglerelative to the inner periphery 31 of the drum 20. During operation, theouter periphery 15 of the helical flights 4 will be worn little bylittle. The helical flights 4 are fastened to the conveyor hub 3 in sucha manner that the angle Υ between the inner edge 16 of the helicalflights and the outer periphery 14 of the conveyor hub is changeable. Inthis way the angles can be changed at a rate according to the rate ofwear of the outer edge 15 of the helical flights 4. This provides theability of the outer edge 15 of the helical flights 4 to be always incontact with the inner periphery 31 of the drum 20. Alternatively, theangle δ can be changed by introducing angular alterations at positionsalong the helical flights 4 other than at the inner edge 16 at the pointof attachment P.

The process of separation that is performed by the illustrated decantercentrifuge is described below:

During operation, the centrifuge rotates about its longitudinal axis ata high speed, which is limited by material strength and criticalvibration frequencies of the design.

In practical terms, the highest safe speed of operation of a rotormounted in fixed bearings is between 50% and 70% of the 1st criticalfrequency of the rotor, depending on the quality of balancing.

As an illustration of these conditions, the following equation gives thecritical frequency of a rotor in principle.

A shaft simply supported at both ends with even thickness distributionand mass m, length between supports 1, sectional moment of inertia I andmodules of elasticity E will exhibit a 1st critical frequency ofvibration ω: ##EQU1##

It is easily seen that ω will increase with increasing E or decreasingm. Further it may be observed that an increase of 1 will cause a ratherlarge reduction of ω.

In real decanter centrifuges, the lowest 1st critical speed is exhibitedby he conveyor, simply because it has the highest mass (=m) inproportion to its stiffness (=EI).

A large improvement of the conveyor, however, will only reveal the nextlimiting factor, which is the combined 1st critical frequency ofvibration of the conveyor and drum.

As the conveyor is supported by bearings relative to the drum, the massof the conveyor will add to the mass of the drum in the equation of 1stcritical frequency of the combined rotor system, and a reduction of theconveyor's weight will therefore have a positive effect on theproperties of the combined rotors as well. It is, however, necessary toimprove the mass/stiffness relationship for the drum, if the fullimprovement of the conveyor is to be taken into advantage.

The centrifuge according to the invention exhibits a drasticallyimprovement of the 1st critical frequency of the conveyor through theapplication of modern light materials for the helical flights andconveyor hub and the added stiffness gained by the introduction of atube 11 (FIG. 1A) of carbon fiber reinforced resin as a backbone in thedesign.

Another important point is that the speed of the drum is limited by thestrength of the material by which the drum wall is manufactured, and avery large proportion of the loads on the material of the drum wallcomes from the weight of the drum wall itself.

The other large load component comes from the liquid pressure on theinside of the drum.

As an illustration of this, look at the following equation giving themaximum safe speed of operation ω for a drum with an outer diameter D,filled with liquid of density ρv, and the drum material has density ρmand maximum allowable stress σ: ##EQU2##

It follows from this that an improvement of the maximum allowable stressθ or an adjacent decrease of the material density m will be needed toincrease ω, and this is exactly the reason behind the design of the drumshell according to the invention, the fiber reinforced material appliedfor the outer shell having a very advantageous relation between strengthand density, resulting in a rotor system of considerably higher 1stcritical frequencies.

A centrifuge of drum diameter 500 mm and a length of 2 m will typicallybe able to reach 5000 rpm.

Sludge to be treated in the centrifuge often consists of small particlesof solids suspended in a liquid, most often water, which fall towardsthe bottom of the container surrounding it by gravity.

By rotating, the centrifuge is capable of producing a field of gravitymany times more forceful than the gravity of earth. In a centrifuge of500 mm diameter and a speed of 5000 rpm, the centrifugal gravity fieldat the inside of the drum will be around 7000 times larger than thegravity of earth.

Through the feed tube 11 (FIG. 1A) and the seal arrangement 35 (FIG. 3)the sludge to be treated is introduced along the rotational axis of thecentrifuge through the hollow shaft 8, further through the hollowconveyor hub 3 to the opening 12, through which it is introduced intothe interior of the drum. When the centrifuge has been in operation fora time long enough to fill up the annular volume 33, the cleaned liquidphase begins to leave the drum by the weir edge 34 provided at the rearend of the drum.

At the same time, the conveyor 2 rotates slowly in relation to the drum3 driven by a transmission (not shown) connected to the conveyor shaft7. This causes the separated solids phase to be moved by the conveyor,as the helical flights are moving along the inside of the drum 20"upward" along the conical sections with the angles α and β (FIG. 2),passing the "waterline" at the end of the annular volume 33 (FIG. 3),finally reaching the solids outlet openings 27, from where the solidsleave the drum and are collected by chutes (not shown).

The speed of the conveyor 2 relative to the drum is dependent on thepitch of the helical flights and, naturally, on the desired dryness ofthe solids, and typical values are between 0.5 and 15 rpm.

The embodiments shown of conveyor, helical flights and decanteraccording to the invention may only be considered as examples. Otherembodiments having properties within the scope of the claims may beprovided. Other materials than polyurethane can be used, as well asstiffening members other than tubes. The angles α, β, γ and δ given byexact values, may take other values as well.

What is claimed is:
 1. A decanter centrifuge for separating solids froma liquid medium, comprising:a drum; and a helical conveyor rotatablymounted within the drum, said conveyor having a conveyor hub and atleast one helical flight joined to the hub, wherein the at least oneflight is made of a single elastomeric material, wherein said centrifugehas a liquid inlet end and a liquid outlet end of larger diameter thanthe inlet end and wherein said drum has an inside curved surface formedof linear sections, said section adjacent the outlet end forming atangential angle of 0 to 8 degrees relative to a longitudinal axis ofthe drum, and said section adjacent the inlet end forming a tangentialangle of 8 to 25 degrees relative to said longitudinal axis.
 2. Adecanter centrifuge for separating solids from a liquid medium,comprising:a drum; and a helical conveyor rotatably mounted within thedrum, said conveyor having a conveyor hub and at least one helicalflight joined to the hub, wherein the at least one flight is made of asingle elastomeric material, wherein said drum comprises an inner metalshell having a wear resistant coating on an inside surface thereof, andan outer shell made from a fiber-reinforced resin.
 3. A decantercentrifuge for separating suspended solids from a liquid medium, whereina helical conveyor is rotatably mounted within a drum and the conveyorincludes a hub and at least one helical flight attached to the hubwherein the hub and the at least one helical flight are integrallyformed in one piece of the same elastomeric material.
 4. The decantercentrifuge according to claim 3, wherein the elastomeric materialcomprises a polyurethane.
 5. The decanter centrifuge according to claim3, wherein the at least one flight further comprises at least one plateor lamella disposed therein to stiffen a portion of the at least oneflight.
 6. The decanter centrifuge according to claim 3, wherein the atleast one flight forms an angle γ relative to an outer periphery of thehub which angle decreases gradually by approximately 90 degrees from afirst end of the hub to a second end of the hub.
 7. The decanter ofclaim 3, wherein the helical conveyor is joined to a drive shaftcomprising a material having a larger modulus of elasticity than theelastomeric material.
 8. The decanter of claim 7, wherein the driveshaft comprises a carbon-reinforced fiber resin and at least a portionof the drive shaft is disposed within the helical conveyor.
 9. Thedecanter centrifuge of claim 3, wherein a portion of the at least oneflight contacts the drum.
 10. The decanter centrifuge of claim 3,wherein the drum has a length and an inner wall comprising at least acylindrical section and a conical section along the length.
 11. Thedecanter centrifuge of claim 10, wherein the conical section comprisesfirst and second conical parts with the first conical part adjacent tothe cylindrical section and having a first surface angle that is lessthan a second surface angle of the second conical part.